Merge pull request riscv#1767 from riscv/1766-integrate-loadstore-pair-for-rv32

Integrating load/store pair for RV32 with the main manual

Author: wmat

src/colophon.adoc

@@ -35,6 +35,7 @@ h|Extension h|Version h|Status
 |*Zihintpause* |*2.0* |*Ratified*
 |*Zimop* | *1.0* | *Ratified*
 |*Zicond* | *1.0* |*Ratified*
+|*Zilsd* | *1.0* |*Ratified*
 |*M* |*2.0* |*Ratified*
 |*Zmmul* |*1.0* |*Ratified*
 |*A* |*2.1* |*Ratified*
@@ -56,6 +57,7 @@ h|Extension h|Version h|Status
 |*Zhinxmin* |*1.0* |*Ratified*
 |*C* |*2.0* |*Ratified*
 |*Zce* |*1.0* |*Ratified*
+|*Zclsd* |*1.0* |*Ratified*
 |*B* |*1.0* |*Ratified*
 |_P_ |_0.2_ |_Draft_
 |*V* |*1.0* |*Ratified*
@@ -78,7 +80,7 @@ h|Extension h|Version h|Status
 
 The changes in this version of the document include:
 
-* The inclusion of all ratified extensions through March 2024.
+* The inclusion of all ratified extensions through February 2025.
 * The draft Zam extension has been removed, in favor of the definition of a misaligned atomicity granule PMA.
 * The concept of vacant memory regions has been superseded by inaccessible memory or I/O regions.
 

src/riscv-unprivileged.adoc

@@ -87,6 +87,7 @@ Jan Gray,
 Gianluca Guida,
 Michael Hamburg,
 John Hauser,
+Christian Herber,
 John Ingalls,
 David Horner,
 Bruce Hoult,
@@ -190,6 +191,7 @@ include::v-st-ext.adoc[]
 include::scalar-crypto.adoc[]
 include::vector-crypto.adoc[]
 include::unpriv-cfi.adoc[]
+include::zilsd.adoc[]
 include::rv-32-64g.adoc[]
 include::extending.adoc[]
 include::naming.adoc[]

src/zfinx.adoc

@@ -97,10 +97,11 @@ operand is zero—i.e., `x1` is not accessed.
 
 [NOTE]
 ====
-Load-pair and store-pair instructions are not provided, so transferring
-double-precision operands in RV32Zdinx from or to memory requires two
-loads or stores. Register moves need only a single FSGNJ.D instruction,
-however.
+Load-pair and store-pair instructions are contained in a separate extension
+(see Section <<sec:zilsd,Extensions for Load/Store pair for RV32>>).
+In case this is not available, transferring double-precision operands in
+RV32Zdinx from or to memory requires two loads or stores. Register moves need
+only a single FSGNJ.D instruction, however.
 ====
 === Zhinx
 

src/zilsd.adoc

@@ -0,0 +1,306 @@
+[[sec:zilsd]]
+== "Zilsd", "Zclsd" Extensions for Load/Store pair for RV32, Version 1.0
+
+The Zilsd & Zclsd extensions provide load/store pair instructions for RV32, reusing the existing RV64 doubleword load/store instruction encodings.
+
+Operands containing `src` for store instructions and `dest` for load instructions are held in aligned `x`-register pairs, i.e., register numbers must be even. Use of misaligned (odd-numbered) registers for these operands is _reserved_.
+
+Regardless of endianness, the lower-numbered register holds the
+low-order bits, and the higher-numbered register holds the high-order
+bits: e.g., bits 31:0 of an operand in Zilsd might be held in register `x14`, with bits 63:32 of that operand held in `x15`.
+
+[[zilsd, Zilsd]]
+=== Load/Store pair instructions (Zilsd)
+
+The Zilsd extension adds the following RV32-only instructions:
+
+[%header,cols="^1,^1,4,8"]
+|===
+|RV32
+|RV64
+|Mnemonic
+|Instruction
+
+|yes
+|no
+|ld rd, offset(rs1)
+|<<#insns-ld>>
+
+|yes
+|no
+|sd rs2, offset(rs1)
+|<<#insns-sd>>
+
+|===
+
+As the access size is 64-bit, accesses are only considered naturally aligned for effective addresses that are a multiple of 8.
+In this case, these instructions are guaranteed to not raise an address-misaligned exception.
+Even if naturally aligned, the memory access might not be performed atomically.
+
+If the effective address is a multiple of 4, then each word access is required to be performed atomically.
+
+The following table summarizes the required behavior:
+
+[%header]
+|===
+|Alignment |Word accesses guaranteed atomic? |Can cause misaligned trap?
+|8B |yes |no
+|4B not 8B |yes |yes
+|else |no | yes
+|===
+
+To ensure resumable trap handling is possible for the load instructions, the base register must have its original value if a trap is taken. The other register in the pair can have been updated.
+This affects x2 for the stack pointer relative instruction and rs1 otherwise.
+
+[NOTE]
+====
+If an implementation performs a doubleword load access atomically and the register file implements writeback for even/odd register pairs,
+the mentioned atomicity requirements are inherently fulfilled.
+Otherwise, an implementation either needs to delay the writeback until the write can be performed atomically,
+or order sequential writes to the registers to ensure the requirement above is satisfied.
+====
+
+[[zclsd, Zclsd]]
+=== Compressed Load/Store pair instructions (Zclsd)
+
+Zclsd depends on Zilsd and Zca. It has overlapping encodings with Zcf and is thus incompatible with Zcf.
+
+Zclsd adds the following RV32-only instructions:
+
+[%header,cols="^1,^1,4,8"]
+|===
+|RV32
+|RV64
+|Mnemonic
+|Instruction
+
+|yes
+|no
+|c.ldsp rd, offset(sp)
+|<<#insns-cldsp>>
+
+|yes
+|no
+|c.sdsp rs2, offset(sp)
+|<<#insns-csdsp>>
+
+|yes
+|no
+|c.ld rd', offset(rs1')
+|<<#insns-cld>>
+
+|yes
+|no
+|c.sd rs2', offset(rs1')
+|<<#insns-csd>>
+
+|===
+
+=== Use of x0 as operand
+
+LD instructions with destination `x0` are processed as any other load, but the result is discarded entirely and x1 is not written.
+For C.LDSP, usage of `x0` as the destination is reserved.
+
+If using `x0` as `src` of SD or C.SDSP, the entire 64-bit operand is zero — i.e., register `x1` is not accessed.
+
+C.LD and C.SD instructions can only use `x8-15`.
+
+=== Exception Handling
+
+For the purposes of RVWMO and exception handling, LD and SD instructions are
+considered to be misaligned loads and stores, with one additional constraint:
+an LD or SD instruction whose effective address is a multiple of 4 gives rise
+to two 4-byte memory operations.
+
+NOTE: This definition permits LD and SD instructions giving rise to exactly one
+memory access, regardless of alignment.
+If instructions with 4-byte-aligned effective address are decomposed
+into two 32b operations, there is no constraint on the order in which the
+operations are performed and each operation is guaranteed to be atomic.
+These decomposed sequences are interruptible.
+Exceptions might occur on subsequent operations, making the effects of previous
+operations within the same instruction visible.
+
+NOTE: Software should make no assumptions about the number or order of
+accesses these instructions might give rise to, beyond the 4-byte constraint
+mentioned above.
+For example, an interrupted store might overwrite the same bytes upon return
+from the interrupt handler.
+
+<<<
+
+=== Instructions
+[#insns-ld,reftext="Load doubleword to register pair, 32-bit encoding"]
+==== ld
+
+Synopsis::
+Load doubleword to even/odd register pair, 32-bit encoding
+
+Mnemonic::
+ld rd, offset(rs1)
+
+Encoding (RV32)::
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits: 7,  name: 0x3,    attr: ['LOAD'], type: 8},
+  {bits: 5,  name: 'rd', attr: ['dest, dest[0]=0'], type: 2},
+  {bits: 3,  name: 0x3, attr: ['width=D'], type: 8},
+  {bits: 5,  name: 'rs1', attr: ['base'], type: 4},
+  {bits: 12, name: 'imm[11:0]', attr: ['offset[11:0]'], type: 3},
+]}
+....
+
+Description:: 
+Loads a 64-bit value into registers `rd` and `rd+1`.
+The effective address is obtained by adding register rs1 to the
+sign-extended 12-bit offset.
+
+Included in: <<zilsd>>
+
+<<<
+
+[#insns-sd,reftext="Store doubleword from register pair, 32-bit encoding"]
+==== sd
+
+Synopsis::
+Store doubleword from even/odd register pair, 32-bit encoding
+
+Mnemonic::
+sd rs2, offset(rs1)
+
+Encoding (RV32)::
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits: 7,  name: 0x23,   attr: ['STORE'], type: 8},
+  {bits: 5,  name: 'imm[4:0]', attr: ['offset[4:0]'], type: 3},
+  {bits: 3,  name: 0x3, attr: ['width=D'], type: 8},
+  {bits: 5,  name: 'rs1', attr: ['base'], type: 4},
+  {bits: 5,  name: 'rs2', attr: ['src, src[0]=0'], type: 4},
+  {bits: 7,  name: 'imm[11:5]', attr: ['offset[11:5]'], type: 3},
+]}
+....
+
+Description:: 
+Stores a 64-bit value from registers `rs2` and `rs2+1`.
+The effective address is obtained by adding register rs1 to the
+sign-extended 12-bit offset.
+
+Included in: <<zilsd>>
+
+<<<
+
+[#insns-cldsp,reftext="Stack-pointer based load doubleword to register pair, 16-bit encoding"]
+==== c.ldsp
+
+Synopsis::
+Stack-pointer based load doubleword to even/odd register pair, 16-bit encoding
+
+Mnemonic::
+c.ldsp rd, offset(sp)
+
+Encoding (RV32)::
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits: 2, name: 0x2,      type: 8, attr: ['C2']},
+  {bits: 5, name: 'imm',    type: 3, attr: ['offset[4:3|8:6]']},
+  {bits: 5, name: 'rd',     type: 2, attr: ['dest≠0, dest[0]=0']},
+  {bits: 1, name: 'imm',    type: 3, attr: ['offset[5]']},
+  {bits: 3, name: 0x3,      type: 8, attr: ['C.LDSP']},
+], config: {bits: 16}}
+....
+
+Description:: 
+Loads stack-pointer relative 64-bit value into registers `rd'` and `rd'+1`. It computes its effective address by adding the zero-extended offset, scaled by 8, to the stack pointer, `x2`. It expands to `ld rd, offset(x2)`. C.LDSP is only valid when _rd_&#x2260;x0; the code points with _rd_=x0 are reserved.
+
+Included in: <<zclsd>>
+
+<<<
+
+[#insns-csdsp,reftext="Stack-pointer based store doubleword from register pair, 16-bit encoding"]
+==== c.sdsp
+
+Synopsis::
+Stack-pointer based store doubleword from even/odd register pair, 16-bit encoding
+
+Mnemonic::
+c.sdsp rs2, offset(sp)
+
+Encoding (RV32)::
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits: 2, name: 0x2,      type: 8, attr: ['C2']},
+  {bits: 5, name: 'rs2',    type: 4, attr: ['src, src[0]=0']},
+  {bits: 6, name: 'imm',    type: 3, attr: ['offset[5:3|8:6]']},
+  {bits: 3, name: 0x7,      type: 8, attr: ['C.SDSP']},
+], config: {bits: 16}}
+....
+
+Description:: 
+Stores a stack-pointer relative 64-bit value from registers `rs2'` and `rs2'+1`. It computes an effective address by adding the _zero_-extended offset, scaled by 8, to the stack pointer, `x2`. It expands to `sd rs2, offset(x2)`.
+
+Included in: <<zclsd>>
+
+<<<
+
+[#insns-cld,reftext="Load doubleword to register pair, 16-bit encoding"]
+==== c.ld
+
+Synopsis::
+Load doubleword to even/odd register pair, 16-bit encoding
+
+Mnemonic::
+c.ld rd', offset(rs1')
+
+Encoding (RV32)::
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits: 2, name: 0x0,       type: 8, attr: ['C0']},
+  {bits: 3, name: 'rd`',     type: 2, attr: ['dest, dest[0]=0']},
+  {bits: 2, name: 'imm',     type: 3, attr: ['offset[7:6]']},
+  {bits: 3, name: 'rs1`',    type: 4, attr: ['base']},
+  {bits: 3, name: 'imm',     type: 3, attr: ['offset[5:3]']},
+  {bits: 3, name: 0x3,       type: 8, attr: ['C.LD']},
+], config: {bits: 16}}
+....
+
+Description:: 
+Loads a 64-bit value into registers `rd'` and `rd'+1`.
+It computes an effective address by adding the zero-extended offset, scaled by 8, to the base address in register rs1'.
+
+Included in: <<zclsd>>
+
+<<<
+
+[#insns-csd,reftext="Store doubleword from register pair, 16-bit encoding"]
+==== c.sd
+
+Synopsis::
+Store doubleword from even/odd register pair, 16-bit encoding
+
+Mnemonic::
+c.sd rs2', offset(rs1')
+
+Encoding (RV32)::
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits: 2, name: 0x0,      type: 8, attr: ['C0']},
+  {bits: 3, name: 'rs2`',   type: 4, attr: ['src, src[0]=0']},
+  {bits: 2, name: 'imm',    type: 3, attr: ['offset[7:6]']},
+  {bits: 3, name: 'rs1`',   type: 4, attr: ['base']},
+  {bits: 3, name: 'imm',    type: 3, attr: ['offset[5:3]']},
+  {bits: 3, name: 0x7,      type: 8, attr: ['C.SD']},
+], config: {bits: 16}}
+....
+
+Description:: 
+Stores a 64-bit value from registers `rs2'` and `rs2'+1`.
+It computes an effective address by adding the zero-extended offset, scaled by 8, to the base address in register rs1'.
+It expands to `sd rs2', offset(rs1')`.
+
+Included in: <<zclsd>>